gistfile1.php

<?php

$dbh = new PDO("mysql:host=localhost;dbname=muratore", "root", "");

$sql = "SELECT count(*) FROM medrithms WHERE username=:username AND contrasena=:contrasena";
$stmt = $dbh->prepare($sql);
$stmt->execute(Array(':username' => $_POST['username'], ':contrasena' => $_POST['contrasena']));

$authenticated = $stmt->fetch(PDO::FETCH_COLUMN);
if ($authenticated) {
  echo "BIENVENIDO";
  echo $username;
} else {
  echo "USUARIO Y/O CONTRASEÑA INCORRECTO";
}